In cystic fibrosis, there is a critical need for better predictors of treatment response. The investigators have identified a panel of white blood cell biomarkers which can be directly measured as a blood test in subjects with cystic fibrosis. These biomarkers predict reduction of airway inflammation and infection more accurately than lung function testing, in patients receiving intravenous antibiotic therapy. In the current study, we hypothesize that this panel of gene biomarkers which can be readily measured from peripheral blood will sensitively predict changes in inflammation when patients receive inhaled antibiotic therapy, specifically Cayston (or inhaled aztreonam lysine). Patients enrolled in the study will have blood drawn before and after a month of inhaled Cayston, in order to test whether genes predict response to Cayston therapy more robustly than do standard measures such as lung function tests.